This invention relates to apparatus for detecting and measuring leaks in heating pipes and the like.
From the German published application No. 2,800,185 it is known to equip measuring stations with thermistors acting as sensors and to lay a measuring cable equipped in this manner along a heating pipeline in order to monitor the occurrence of any leakage point causing heat loss and, if necessary, to be able to exactly locate such leakage points. This is carried out by measuring in the control centre any variation of the temperature gradient between two adjacent measuring stations and evaluating them for fault detection.
The absolute value of the ambient temperature at the respective measuring stations or measuring points during each individual measuring operation is of small importance since it changes very often due to the effects of aging and weather, e.g. changing intensities of sunshine. One therefore proceeds in such a manner that the newly obtained values measured at the respective measuring points, one after the other, are compared with the so-called set values assigned to these measuring points. These set values are adapted to the generally changing ambient conditions in that measurements are taken at the measuring points over and again and are stored as new set values for the next period when such deviations from the old set values are determined as having been caused by a "global" temperature change, i.e. over large sections of the heating pipeline.
It is possible to apply the inventive measuring arrangements to other fields of application when other transducer components or circuits are inserted instead of the thermistors, into each measuring point. These substituted components or circuits may change their electric resistance dependent, for instance on the ambient pressure, the ambient degree of brightness, the ambient humidity, etc. It is also by no means necessary that all measuring points be equipped with the same sensors. By way of example, in a building complex comprising numerous rooms, it is possible to install an inventive measuring arrangement and thereby to monitor in each room the temperature, humidity, the operational state of the lights, the development of smoke, etc., in which case a measuring point is provided in each of the rooms for each of these physical ambient parameters and the measuring point is provided with an appropriate sensor. All these measuring points can be connected to one another in series by the cable of the measuring arrangement and can be scanned one after the other by the control and measuring centre.
In many cases the set values used for the purpose of comparison with the individually measured values may also be determined in a manner other than has been described hereinbefore with respect to temperature.
Thus, for example, for checking the state of operation of the room lights, a set value is fixed for the daytime or hours of work which corresponds to a higher degree of brightness and a set value is fixed for the night which corresponds to a lower degree of brightness, in order to thus obtain an indication of where it has been forgotten to turn out the light at night.
For determining the development of smoke, on the other hand, a light source of constant brightness is arranged in front of a photosensitive sensor and a single fixed set value is provided, so that an inadmissible reduction of the transparency of the air in the room in question is signaled, when the measured value surpasses or falls below said set value.
In the known arrangement the control circuit of each measuring point substantially comprises two monoflops, the first of which is in each case triggered by the output impulse of the second monoflop of the preceding measured point after which it emits an output pulse with a predetermined time length which closes the switch arrangement for the period it appears at the control input of this switch arrangement connecting the sensor of this measuring point to a wire conducting the current to be impressed and which triggers the second monoflop of the respective measuring point by means of its trailing edge. The second monoflop produces by means of a short closing of a second switch a feedback to the control and measuring centre indicative of the end of the measuring procedure, and it then triggers the first monoflop of the next measuring point.
The monoflops of all measuring points of the known measuring arrangement are therefore closed together to form a long single chain. The control and measuring centre can only trigger the first monoflop of the first measuring point, after which this trigger pulse spreads like a wildfire through the monoflop chain without the centre having any further possibility to intervene.
In practical operation, a whole series of problems can result from this step-by-step switching from one measuring point to the next without the control of the control and measuring centre. If, by way of example, at a certain measuring point a measured value stepping out of line is to be checked, the complete monoflop chain has to be clocked through again in order to obtain a further value from this one measuring point.
A further problem resulting from the fact that the control and measuring centre cannot intervene, is seen in that it is extremely difficult to branch off the known measuring system. The second monoflop of the measuring point preceding such a branching would trigger the first monoflops of the next measuring points following in each branch almost at the same time so that several measured values would have to be fed back simultaneously to the centre. In the case of a single Y-shaped branch the difficulties arising can be solved, but they can no longer be solved in the case of a system which is branched to any desired extent and at any number of points.
Finally, the fact that the control and measuring centre cannot intervene during the course of the measurements at the individual measuring points, is particularly disadvantageous if, as is preferably the case, the switch arrangements of the measuring points are formed by contactless semiconductor switches, e.g. field effect transistors. As is known per se, such "analog switches" can be switched to and fro between an almost unlimitedly high resistance value appearing between their source-drain connections ("switch open") and a resistance value near zero ("switch closed"), by feeding a suitable pulse-shaped control signal to their control inputs. During the rising and falling times of the control signal pulse, a continuous transition occurs, however, between the two marginal resistance values so that measurement of the resistance of a sensor connected in series to such a switch is only appropriate in the steady state, i.e. when the switch is completely closed, but not during the rising or falling times of the control pulse edges.
Since the control pulses in the case of the known system are in each case produced by the first monoflop of the respective measuring point, it is very difficult to allow the control and measuring centre to operate such that measurements are always determined only in the steady state.
It is therefore the object of the invention to provide a measuring system of the type set forth at the outset, in which the control and measuring centre is better able to influence the measuring procedures running off one after the other at the measuring points.